The present invention relates to a gap-providing ferrite core half and a method for producing it.
One example of conventional gap-providing ferrite core halves is shown in FIG. 21. An E-type ferrite core half 63 comprising two outside leg portions 61 and one center leg portion 62 which is shorter than each outside leg portion 61 by a distance called a gap "G." The gap is provided for the purpose of controlling direct-current superposition properties of the ferrite core.
A conventional ferrite core 220 with a gap for transformers, choke coils, etc. as shown in FIG. 22 is formed by combining a gap-providing E-type ferrite core half 63 as shown in FIG. 21 and an E-type ferrite core half 64 with no gap-providing recess such that legs of both core halves abut each other, with a coil disposed around the aligned center leg portions. Such a combination of core halves is called "EE-type core assembly" or simply "EE-type core."
In addition to an EE-type ferrite core, there is an EI-type ferrite core comprising a gap-providing E-type ferrite core half as shown in FIG. 21 and a flat I-type ferrite core half. Though the gap-providing E-type ferrite core half shown in FIG. 21 has three legs, there may be ferrite core halves having two or four or more legs.
The provision of a gap-providing recess to a ferrite core half has been achieved by forming a ferrite core half 230 having outside leg portions 65a, 65a and a center leg portion 65b of the same length (shown in FIG. 23), sintering it, grinding flat an end surface 66 of each leg 65, and further grinding an end surface 66 of a center leg portion 65b only. Also, as shown in FIG. 24, with an integral diamond grinder 67 rotatable around an axis 69 and having a grinding layer 68 disposed on a stepwise surface of the grinder 67 such that the grinding layer 68 is complementary with the legs of a ferrite core half to be ground, abutting surfaces of the legs of the E-type ferrite core half can be ground.
FIG. 25 is a perspective view showing another example of the conventional gap-providing ferrite core half, and FIG. 26 is a front view showing the gap-providing ferrite core half of FIG. 25. This gap-providing ferrite core half is a U-type ferrite core half 101 with two legs 102, 103, one leg 103 being shorter than another leg 102 by a distance called a gap "G" for the purpose of controlling direct-current superposition properties of the ferrite core. This gap-providing ferrite core half 101 is combined with a U-type ferrite core half 104 of the same shape without a gap-providing portion to form a transformer with a coil wound around one leg.
In this conventional gap-providing U-type ferrite core half, only one leg thereof is provided with a gap-providing recess as shown in the drawings. In this case, end surfaces of two legs are ground flat to a high precision, and an end surface of only one leg is further ground to a predetermined depth to provide a gap-providing recess.
Since a gap dimension greatly affects the properties of these ferrite core assemblies with gaps, it is important to provide the gap dimension with a high precision. However, since a particular leg end is ground after all leg ends are ground flat in the conventional gap-providing method, two grinding steps are needed, making the production cost high. Further, since the grinding of the center leg portion is conducted at a different work position from the previous one at which the outside leg portions are ground, it is difficult to provide a gap-providing recess with a high grinding precision.
Though it is possible to grind both outside leg portions and a center leg portion simultaneously with an integral grinder as shown in FIG. 24, grinders having different grinding surfaces should be prepared for ferrite core halves with different gap-providing recesses, making the types of the grinders stocked extremely many. For this reason, the conventional methods suffer from high production cost due to large numbers of grinding steps and difficulty in fine control of gap dimension.
If a sufficient number of steps are used with an expensive grinding machine, a gap dimension with a high precision may be obtained. However, this would be low in productivity and high in cost.
As shown in FIG. 27, a ferrite core assembly with a gap may be constituted by two ferrite core halves 105, 105 each having no gap-providing recess, with one or two spacers 106, 106 inserted between the ends of aligned legs.
In case where a spacer is inserted between the abutting ends of legs to provide a gap-providing recess, there are the following disadvantages:
(1) The step of inserting a spacer is necessary; PA1 (2) Varieties of spacers should be prepared depending on the gap dimensions; PA1 (3) Particularly in the case of a small gap dimension, an extremely thin spacer is needed, making its handling difficult.
As mentioned above, there have not been gap-providing ferrite core halves which can be produced precisely at a high productivity and low cost.